Plasma processing apparatus and method for the plasma processing of substrates

ABSTRACT

A plasma processing apparatus ( 30, 50 ) comprises a process chamber with process chamber walls ( 35 ), process gas inlet means and process gas distribution means in said process chamber, exhaust means for removal of residual gases and a substrate mount ( 34 ) for a substrate ( 33 ). In a first embodiment a conductive plate ( 51 ) is arranged within said process chamber, electrically connectable with at least one RF power source ( 39 ) facing said conductive plate ( 51 ), exhibiting a pattern of openings and arranged at a distance to a backside wall ( 53 ) of said process chamber so that a process gas delivered to a gap ( 55 ) between the conductive plate ( 51 ) and said backside wall ( 53 ) does not ignite a plasma in the gap ( 55 ) during operation. In a second embodiment a first and second electrode are arranged within said process chamber adjacent each other with a gap in-between. The first electrode is connectable to a RF power source and the second electrode is connected to ground. The second electrode exhibits a pattern of openings and is arranged at a distance such that a process gas delivered to said gap does not ignite a plasma during operation.

The present invention refers to a plasma processing apparatus or system with improved (low energy) ion bombardment properties and to a method for processing substrates in an apparatus of said kind. Plasma processing refers to deposition- and/or etching processes, heating, surface conditioning and other treatments of substrates.

BACKGROUND OF THE INVENTION

Many plasma processing systems known in the art are construed according to the so-called parallel plate reactor principle. A plasma processing apparatus of that kind is shown in FIG. 1. It comprises a process chamber 7 with walls defining an enclosure, a first plane electrode 1, a second plane electrode 2, both arranged within said process chamber 7, electrically connectable with at least one RF power source. Electrodes 1 and 2 define a plasma generation region 6. A substrate 5 to be processed is placed on a substrate holder or, as shown, directly on one of the electrodes. Thus the substrate is exposed to the effects of the plasma during processing. Process gas inlet means 3 as well as exhaust means 4 for removal of residual gases are shown schematically in FIG. 1. Process gas distribution means have been omitted.

Typically thin film silicon layers (amorphous, nano/microcrystalline material etc.) and its alloys with C, N, O etc. are deposited by the PECVD (plasma enhanced chemical vapor deposition) technique using this parallel plate setup and capacitive RF power coupling. Usually the substrates are placed on a grounded electrode whereas the other electrode serves as the RF powered electrode and as a gas distribution shower head (process gas distribution means). Using such a setup, homogeneous depositions of amorphous and nano/microcrystalline layers over large areas in the square meter range have been obtained successfully.

RELATED ART

In the classical parallel plate configuration the maximal possible ion energy is correlated in a well known manner (Köhler et al. J. Appl. Phys. 57 (1985), p. 59 and J. Appl. Phys. 58 (1985), p. 3350) with the applied RF peak/to/peak voltage which is closely correlated with the RF power applied. Ion bombardment with ions accelerated towards a substrate over a certain threshold voltage value in the process chamber creates defects in the deposited bulk material and damages sensitive interfaces and, hence, deteriorates the material quality and interface performance. Several attempts using VHF and/or high pressure deposition regime, triode configuration etc. were carried out to reduce this bombardment especially for the deposition of microcrystalline layers. Triode configurations lead to excellent material, but reduce the deposition rates because a recombination of radicals in the zone between grid and substrate takes place. Increased plasma excitations frequencies into the VHF/UHF regime reduces the necessary peak-to-peak voltage for a given power of the plasma, but again the ion-bombardment increases with the RF power and, hence, can not be controlled independently and adjusted alone.

An efficient manner to control the ion bombardment independently of the RF power could be realized by placing the substrates on a floating electrode. In this case only the floating potential, which is much lower than the plasma power dependent plasma potential, would accelerate ions towards the substrate leading to a considerable reduction of the maximum possible ion energies. However, simply allowing the grounded electrode to become electrically floating in the parallel plate setup would remove most of the electrical ground especially at large area applications due to the absence of the electrode ground potential. Only the grounded chamber walls in electrical contact with the plasma would remain as ground for the plasma.

There are Prior Art applications addressing this problem. FIG. 2 (cited from U.S. Pat. No. 7,090,705) shows a plasma processing apparatus comprising an electrode configuration facing a substrate 21 with a first electrode 24 and a second electrode 22 spaced apart by insulators 23. The second electrode 22 is arranged in a stripe pattern in a parallel plane to the first electrode 24, resulting in a structure of parallel trenches 26 with a part of the first electrode 24 acting as the trench base 27 and electrode 22 acting as trench shoulder. Electrical power, preferably RF power 25 is being applied between electrodes 22 and 24, such that plasma is generated in the trench 26 and adjacent to electrode(s) 22. Technically this trench 26 could be described as elongated cavity and might even be using a hollow cathode effect.

Process gas is being delivered to the trenches 26 via holes in the trench base 27. The smooth and even distribution of the process gas is essential for an effective operation and in order to achieve a homogeneous result of the processing, e.g. layer deposition or etching step.

The stripe pattern of electrode bars allows control of the plasma generation areas, however, the manufacturing effort is considerable, since the elements of second electrode 22 and insulators 23 have to be assembled individually; furthermore many drill holes are required in order to achieve a smooth and even process gas distribution.

For large area plasma deposition systems based on the parallel plate reactor principle the standing wave phenomenon will occur. This is due to the fact that with increasing RF/VHF frequency (>13.56 MHz and electrode diameters >1 m) the free space wavelength decreases and thus a standing wave in the reactor develops, starting from the point of delivery, the connecting point of the RF power to the electrode. The design shown in FIG. 2 tries to avoid this by multiple points of delivery for the RF power, one per electrode 22. However, this means that a costly and elaborate wiring is necessary in order to achieve at least partial independence of said phenomenon. The cost for such wiring will increase with increasing size of the electrode according to FIG. 2. However, the design according to FIG. 2 does not resolve the problem of the standing wave completely. Standing waves may still occur along the parallel trenches 26, especially for large area electrodes.

A further general problem of the parallel-plate reactor design, especially for large surfaces, lies in the fact, that the electrical current flow properties are not equal for all areas of the electrode(s). Regions close to the edges of the electrodes perceive “more” effective anode area than central areas of the electrodes due to the fact that the walls of the process chamber usually have ground connection and therefore will also act as an anode. A design as described in FIG. 2 cures this problem at least partially, because electrode surfaces are distributed and arranged in close relationship so this inhomogeneity, especially for the central regions of large areas electrodes will be reduced.

It is therefore an objective of the invention to avoid the disadvantages of the Prior Art designs, to demonstrate a lightweight, costeffective and scalable design of an electrode to be used in a plasma processing apparatus. It is further an object of the invention to provide for a method for plasma treatment of substrates with increased flexibility in terms of deposition rate, influence on crystallinity of deposited layers and layer homogeneity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a parallel plate reactor design (simplified) according to Prior Art.

FIG. 2 shows a reactor as described in Prior Art document U.S. Pat. No. 7,090,705

FIG. 3 shows a first embodiment of the invention in side view

FIG. 4 shows a top view on a perforated electrode according to the invention

FIG. 5 shows photographs made from a small scale reactor following the principles of the invention.

FIG. 6 shows a second embodiment of the invention in side view.

SOLUTION ACCORDING TO THE INVENTION

A plasma processing apparatus 30 comprises a process chamber with process chamber walls 35, process gas inlet means and process gas distribution means in said process chamber, exhaust means for removal of residual gases, at least a first and second electrode 31, 32 arranged within said process chamber, electrically connectable with at least one RF power source 39. Further a substrate mount 34 for a substrate 33 to be processed is being provided for. The second electrode 32 exhibits a pattern of openings and is arranged at a distance to the first electrode 31 so that a process gas delivered to the gap 38 between the electrodes 31, 32 does not ignite a plasma in the gap 38 during operation.

In an alternative embodiment a plasma processing apparatus 50 comprises a process chamber with process chamber walls 35, process gas inlet means and process gas distribution means in said process chamber, exhaust means for removal of residual gases, at least a perforated (conductive) RF plate 51 arranged within said process chamber, electrically connectable with at least one RF power source 39. Further a substrate mount 34 for a substrate 33 to be processed is provided for. The RF plate 51 exhibits a pattern of openings and is arranged at a distance to backside wall 53 so that a process gas delivered to a gap 55 between the RF plate 51 and backside wall 53 does not ignite a plasma in the gap 55 during operation.

A method for plasma processing of substrates comprises introducing a substrate 33 into a plasma processing apparatus 30, placing the substrate 33 on a substrate holding means 34 facing an electrode 32 with openings 36 therein, setting appropriate process conditions (pressure, process gases, temperature) and igniting localized plasmas 37 in the openings 36 of said electrode 32 and processing said substrate.

DETAILED DESCRIPTION OF THE INVENTION

The inventive solution is based on a modified electrode configuration for a plasma processing system (or plasma reactor) 30 as shown as a first embodiment's side view in FIG. 3.

A grounded plate 32 with holes or openings 36 (perforated ground plate) is arranged adjacent to an electrode 31 operatively connectable to a RF power source 39. Gas inlet means (not shown) are preferably provided for delivery of process gases into the gap 38 between grounded plate 32 and powered electrode 31. A commonly used gas shower-head can be also implemented in this setup by e.g. providing for holes in the powered electrode 31 located opposite the holes of the grounded electrode. The distance between said electrode 31 and grounded plate 32 is chosen such that in the gap 38 between electrode 31 and plate 32 no plasma will ignite (effective dark-space shielding). The distance can vary depending on the voltage and the RF frequency applied and the gas pressure and nature of gas set in the gap 38. Technically the distance can be set by the use of isolating spacers, such as ceramic screws, this way defining the separation distance between electrodes 32 and 31. In one embodiment the distance between the RF electrode 31 and the grounded plate 32 is arranged to be at around 1 to 3 mm.

By disposing the grounded plate 32 in vicinity to electrode 31 the plasma is forced to burn in the holes 36 of the electrically grounded plate 32. This way localized plasma(s) 37 can be generated. The spatial distribution of radicals produced in the burning localized plasma(s) 37 and the ratio of grounded/powered electrode area can be adjusted by a proper design of the distribution of the holes, the holes' diameters, the shape of the holes and their area density. This way it is additionally possible to achieve on substrate 33 a uniform layer with an excellent thickness uniformity and superior etch rate uniformity, respectively.

Moreover, the hollow cathode principle can be extended to these holes to even further enhance the plasma dissociation. In this case a preferred embodiment would comprise holes with a diameter of 1-30 mm, preferably 8-15 mm, further preferred 10 mm. The thickness of the perforated ground plate/electrode 32 can be selected between 1-15 mm, preferably 5-15 mm and 10 mm further preferred. Such “thick” electrode could then also be used to act as gas distribution means to allow dosing of process gas to the gap between electrodes 31 and 32.)

A substrate 33 can be located on a substrate holder 34, which again can be designed to be electrically floating, i.e. separated from the perforated ground electrode plate, the powered RF electrode 32 and the process chamber walls 35 by an appropriate distance (approx. 5 mm to 100 mm, depending on pressure, hole geometry etc.).

In a further embodiment an additional, separate, second RF power supply 40 can be connected between substrate holder 34 and ground, which will allow an independent bias and, hence, control of the moderate but sometimes still beneficial ion bombardment. By this crystallinity and/or density of a deposited layer can be varied to a larger degree, because the ions generated by the localized plasmas can be directed towards the substrate.

FIG. 4 shows a top view on perforated ground plate/electrode 32 with holes 36. Here a regular pattern of holes is being shown; however, the density of holes could be varied in order to compensate e. g. edge effects, standing wave effects etc. Electrode 32 can be manufactured from a single sheet of metal with a few mm thickness. Then, the holes can be easily laser-cut, which will also ease variations of the diameter of the holes and avoid the efforts of drilling including the problem of drill-breakage.

The products to be processed with an inventive plasma processing system include large area (>1 m²), essentially flat substrates, such as solar panels on glass and glass ceramics as well as other material (plastic, stainless steel), further display panels for TFT or other applications. The range of applications includes deposition and/or etching processes, heating, surface conditioning and other treatments of aforementioned substrates. Process gases useful in etching processes are CF₄, SF₆, Cl₂, HCl, BCl₃, O₂ or others. For deposition of layers, especially semiconductor layers, gases like

Silane SiH₄, disilane, dichlorosilane, SiF₄, GeH4 etc. plus eventual dopants, ammonium NH₃, nitrogen N₂, Hydrazin etc. (for silicon nitride layers), N₂O, CO₂ and O₂ etc. (for silicon oxide layers), hydrogen H₂ (as dilutant for many deposition processes) are used with preference.

The plasma processing apparatus according to the invention can be used for RF/VHF frequencies between several hundred kHz to several hundred MHz. By far preferred are the industrially used 13.56 MHz plus its harmonics like 40 MHz, and more. A small-scale reactor in operation is shown in FIG. 5 a) and b), the localized plasmas are visible as bright spots.

In a third embodiment, the inventive principle can be further simplified, as shown in FIG. 6. Instead of using a powered electrode 31 in conjunction with a grounded, perforated plate 32 (FIG. 3) a perforated (conductive) RF plate 51 is being used. Plate 51 is connected to RF power source 39. The process chamber wall 53 just behind will act through the holes as anode. The distance between RF plate 51 and backside wall 53 (=part of process chamber walls 35) is chosen such that in the gap 55 no plasma will ignite (effective dark-space shielding). However, this space will, in connection with a gas distribution system (not shown) provide each localized plasma with process gases.

Localized plasmas 37 will ignite in the openings of RF plate 51. The perforated RF plate 51 allows compensating the effect of the standing wave effect in the same way as grounded plate 32 for the embodiment in FIG. 3 by adjusting size, density and configuration of the localized plasmas. The options and limitations cited above can be seamlessly applied also for this embodiment, unless indicated to the contrary.

In this third embodiment a plasma 52 adjacent to perforated RF plate 51, facing the processing region 54 might occur. This is due to the fact that not only between plate 51 and backside wall 53 an electric field is being established, but also between powered plate 51 and the other regions of process chamber walls 35.

A second RF power supply 40 can be advantageously connected between substrate holder 34 and ground, which will allow an independent bias and, hence, control of the moderate but sometimes still beneficial ion bombardment as described above for FIG. 3.

In order to avoid that substrate 34 is affected in unwanted manner by ion bombardment it should be taken care that no or at least minimal capacitively coupling occurs between substrate holder 34 and the residual process chamber wall 35.

Further Advantages of the Invention

By using a reactor setup according to the invention's first to third embodiment, one RF generator is used to generate locally the reactive radicals. This RF voltage generates a high bombardment, but this bombardment will be directed onto the RF electrode 31 or 51 and its corresponding ground connection (perforated ground plate 32 and backside wall 53) and not to a floating electrode or substrate holder 34 where the substrate 33 is placed and the soft deposition conditions should occur. Technically this means plasma generation and creation of radicals are being decoupled from the deposition, especially if a separate (RF) substrate bias voltage 40 is being used. Thus, the deposition rate can be increased by using higher frequencies and/or higher voltages without risking damage to the substrate.

Since the main and strong bombardment will not take place on the substrate the latter will be less heated up by the bombardment and, hence, will remain less affected by heating up during the deposition.

In contrast to the design of FIG. 2, where the standing wave effect is partially avoided by complex electrical wiring and arrangement of segmented electrodes 22, the inventive design takes the phenomenon into account and allows to compensate it by the hole pattern design as described above (diameter, arrangement, density etc. . . . ). This allows keeping the wiring simple, using only one or 2-4 connection points to one electrode and at the same time the constructional and assembly efforts for the electrodes are minimized.

It is possible to extend (scale up) the inventive design to an arbitrary size (very large area reactors, >3 m²) without an immense technological constructive effort. 

1. A plasma processing apparatus (30) comprising a process chamber with process chamber walls (35), process gas inlet means and process gas distribution means, exhaust means for removal of residual gases, at least a first and second electrode (31, 32) arranged adjacent to each other thus forming a gap (38) in-between, at least one RF power source (39) electrically connectable with said electrodes (31, 32), and a substrate mount (34) for a substrate (33) facing second electrode (32), characterized in that the second electrode (32) exhibits a pattern of openings (36) and is arranged at such a distance to the first electrode (31) that a process gas delivered to the gap (38) between the electrodes (31, 32) does not ignite a plasma in the gap (38) during operation and wherein the second electrode is grounded.
 2. An apparatus according to claim 1, wherein the distance between first electrode (31) and second electrode (32) is essentially between 1 to 3 mm.
 3. An apparatus according to claim 1 or 2, wherein the second electrode is designed as a plate with a thickness between 1-15 mm.
 4. An apparatus according to claims 1 or 2, wherein the substrate mount (34) is connectable to a second RF power supply (40).
 5. A plasma processing apparatus (50) comprising a process chamber with process chamber walls (35), process gas inlet means and process gas distribution means in said process chamber, exhaust means for removal of residual gases, a conductive plate (51) arranged within said process chamber, electrically connectable with at least one RF power source (39) and a substrate mount (34) for a substrate (33) facing said conductive plate (51), characterized in that the conductive plate (51) exhibits a pattern of openings and is arranged at a distance to and facing a backside wall (53) of said process chamber so that a process gas delivered to a gap (55) between the conductive plate (51) and said backside wall (53) does not ignite a plasma in the gap (55) during operation.
 6. An apparatus according to claim 5 wherein the substrate mount (34) is connectable to a second RF power supply (40). 